A spark plug is installed to, for example, an internal combustion engine (engine), and used for igniting an air-fuel mixture within a combustion chamber. In general, the spark plug includes an insulator having a shaft hole, a center electrode that is inserted through a leading end side of the shaft hole, a metal shell that is disposed in an outer periphery of the insulator, and a ground electrode that is joined to the metal shell and forms a spark discharge gap between the center electrode. The metal shell and the insulator are fixed by engaging a stepped portion formed at an outer periphery of the insulator with a stepped portion formed at an inner periphery of the metal shell and then bending a rear end of the metal shell inward in a radial direction.
Further, for the purpose of improving a corrosion resistance, a nickel plating layer may be disposed on a surface of the metal shell on which the ground electrode is welded (for example, refer to JP-A-2002-184552, “Patent Reference 1”).